In video system applications, a picture is displayed on a television or computer screen by scanning an electrical signal horizontally across the screen one line at a time. The amplitude of the signal at any one point on the line represents the brightness level at that point on the screen. A video frame contains the necessary information from the lines that make up the picture and from the associated synchronization (sync) signals to allow a scanning circuit to trace the lines from left to right and from top to bottom in order to recreate the picture on the screen. This information includes the luma (Y), or brightness, and the chroma (C), or color, components of the picture. There may be two different types of picture scanning in a video system. The scanning may be interlaced or it may be non-interlaced or progressive. Interlaced scanning occurs when each frame is divided into two separate sub-pictures or fields. The interlaced picture may be produced by first scanning the horizontal lines that correspond to the first field and then retracing to the top of the screen and scanning the horizontal lines that correspond to the second field. The progressive or non-interlaced picture may be produced by scanning all of the horizontal lines of a frame in one pass from the top to the bottom of the screen.
The luma (Y) and chroma (C) signal components that represent a picture are modulated together in order to generate a composite video signal. Integrating the luma and chroma video elements into a composite video stream facilitates video signal processing since only a single composite video stream is transmitted. Once a composite signal is received, the luma and chroma signal components must be separated in order for the video signal to be processed and displayed as a picture on the screen. A comb filter may be utilized for separating the luma and chroma video signal components. For example, a television may be adapted to receive a composite video input but the chroma and luma video components have to be separated before the television can display the received video signal.
FIG. 1A is a diagram illustrating the generation of a conventional composite video signal. Referring to FIG. 1A, adding the chroma signal component 102 and the luma signal component 104 produces a composite video signal 106. The luma signal component 104 may or may not increase in amplitude in a stair step fashion. The chroma signal component 102 may comprise a color difference component U that is modulated by, for example, a sine signal with a 3.58 MHz frequency, and a color difference component V that is modulated by, for example, a cosine signal with a 3.58 MHz frequency. The modulation scheme may be selected so that it provides quadrature modulation between the U and V color difference components. An exemplary composite video signal 106 may be a composite video signal with burst and syncs (CVBS).
FIG. 1B is a diagram illustrating the position of color burst and active video in a conventional composite video signal. Referring to FIG. 1B, a portion of the composite video signal 108 may be a color burst 110 and a different portion may be the active video signal 112. The color burst 110 may comprise a brief sample of, for example, eight to ten cycles of unmodulated color subcarrier which have been inserted by an NTSC or PAL encoder onto the back porch of the composite video signal to enable a decoder to regenerate the color subcarrier from it. The active video portion 112 of the composite video signal 108 contains the luma and chroma signal components of the picture or image.
FIG. 1C is a graphical diagram illustrating the phase relationship of modulated chroma signals in contiguous composite video signal frames. The chroma signal component in the active video portion of an NTSC composite video signal may modulated at such a frequency that every line of video in a video frame is phase-shifted by 180 degrees from the previous line. Referring to FIG. 1C, the bottom frame, the current frame, and the top frame are contiguous composite video frames and the (M−1) video line, the M video line, and the (M+1) video lines are contiguous video lines within the video frame, where M corresponds to any current line which may have a previous line and a next line adjacent to it. The “bottom frame” may correspond to the frame that is currently being received while the “current frame” and the “top frame” may correspond to frames that have been delayed by one and two frames respectively. The M video line in the “current frame” is phase-shifted by 180 degrees from the (M−1) video line in the “current frame” as well as from the (M+1) video line in the “current frame.” Similarly, the M video line in the “bottom frame” is phase-shifted by 180 degrees from the (M−1) video line in the “bottom frame” as well as from the (M+1) video line in the “bottom frame.” In addition, since the fields are at a frequency rate of 59.94 Hz, there is a 180-degree phase shift between two adjacent frames, for example, the “current frame” and the “top frame.” Correspondingly, the M video line in the “current frame” is 180 degrees phase-shifted from the M video line in the “top frame.” In a PAL composite video signal, adjacent video lines and adjacent frames may have a 90-degree phase shift, requiring a two line or two frame delay in order to obtain video lines or frames with a 180-degree phase shift.
In conventional video processing, there are three ways to separate the luma and chroma video components received in a composite video signal—by utilizing a notch filter, by combing vertically or by combing temporally. During separation of the luma and chroma signal components, there are three bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth, horizontal bandwidth, and/or temporal bandwidth.
The first way to separate the luma and chroma video components is by utilizing a notch filter. Since the components in a chroma signal are modulated at 3.58 MHz, a notch filter that is set at 3.58 MHz may be utilized. The notch filter, however, reduces the horizontal bandwidth in the output video signal and as a result the luma video component is increased. A comb filter delays a prior horizontally scanned line in order it compare it with a currently scanned horizontal line. Combing vertically may also be utilized to separate the luma and chroma video components. Combing vertically may be achieved in three different ways—the current line may be combed with the previous and the next line, the current line may be combed with the line just before it, or the current line may be combed with the line just after it. The vertical combing is performed spatially, i.e., only within two fields at a time and without any temporal combing. During combing in the “current frame,” for example, if the current line is added to the previous line, the chroma content cancels out and two times the luma content is obtained. On the other hand, if the previous line is subtracted from the current line, the luma content cancels out and two times the chroma content is obtained. In this way, luma and chroma content may be separated from the composite video signal for further processing. In addition, the vertical combing results in a reduced vertical bandwidth.
A third way to comb a composite signal is to comb temporally. Combing temporally comprises combing between two adjacent or contiguous frames, for example, the “current frame” and the “bottom frame” or the “current frame” and the “top frame.” Further, temporal combing is characterized by a reduced temporal bandwidth. The luma and chroma components may be separated by utilizing the same addition and subtraction methodology between a current line and a previous line, which is employed by vertical combing.
While 2-D comb filters may be adapted to process successive scan lines for a single field of a video frame, 3-D comb filters may be adapted to process scan lines that are taken from successive video frames. In general, for 3-D comb filtering, if there is motion between the successive video frames, a 3-D comb filter must revert to 2-D comb filtering. Motion includes color changes and image movement between frames. Accordingly, the 3-D comb filter may be required to buffer at least one frame in order to determine whether there is motion between the buffered frames. In an instance where there are color changes or image movements between the buffered frames, the corresponding Y/C components for the buffered frames will be different and the results of combing would be incorrect.
Since the 3-D comb filter may be required to buffer at least one frame of video data, several complete frames of video data must be stored in buffers as opposed to just 2 or 3 lines which are required by 2-D comb filters. Accordingly, 3-D comb filters require a large or significant amount of video memory and excessive memory processing bandwidth requirements. This large memory and excessive memory processing bandwidth requirements, along with the necessary motion detection processing, increases the cost associated with 3-D comb filter solutions.
Three-dimensional (3D) combing algorithms typically operate on video samples that are temporally separated by a video frame, for example, the bottom frame, the current frame, and the top frame in FIG. 1C. These temporally separated video samples or video frames must be exactly aligned in order to prevent the formation of artifacts in the displayed picture due to misalignment. Alignment of the temporally separated video frames is generally achieved by delaying the frames. The amount of the delay required to align these frames generally varies depending on the video processing standard and/or processing scheme employed. For example, the amount of delay system required for NTSC and PAL standards and for progressive scan input video may vary between the standards. Furthermore, although some signals may conform in some respects to a particular standard, these signals may vary outside the specified ranges permitted by the particular standard. These signals that vary outside the specified ranges permitted by the standard may be referred to as non-standard signals. For example, a non-standard signal, which may be part of a data stream, may have frame lengths that vary outside the ranges permitted by a specific standard or these signals in the data stream may violate the relationship between a specified line length and the subcarrier frequency.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.